Many current sensing platforms use thin films made of membranes, polymers, solgels, etc., to determine identity and/or concentration of analytes of concern. These thin film technologies have inherent limitations with regard to selectivity. In particular, the thin films are not analyte specific. Therefore, the sensors incorporating such thin films respond to classes of compounds in the same manner. No additional specific information is provided by the transduction mechanism of these thin films.
Thus, concentrations are often reported as equivalent values, where a specific concentration is assumed to be of a particular compound. For example, all aromatic organic compounds are often reported as "toluene equivalent" concentrations. For some compounds, this equivalent is a fairly accurate representation of the concentration of the analyte, while for other compounds, it can be inaccurate by an order of magnitude or more.
In order to improve the discrimination ability of the current sensor technologies, one proposed solution has been to use arrays of sensors where multiple devices of the same sensor type are coated with different thin films. However, the information provided by each individual sensor is not unique when compared to the response of the other sensors. In other words, the information provided by additional sensors represents an incremental increase over the information provided by all previous sensors. Building a calibration model based on the response signal from all of the sensors after exposure to a particular analyte improves the overall selectivity of the sensing system, but also is very difficult. Further, the potential for failure of this sensing system goes up geometrically in accordance with the number of individual sensors in the array.
In order to obtain high selectivity using multiple devices with different thin films, a large array of sensors is required. This poses a problem with respect to fabrication and calibration, as well as long term stability and use. It is difficult to cast equivalent and reproducible films, especially at a reasonable cost.